You should search the net to see if you can find a typical heated slab detail, for reference, to provide some insight for reinforcing, I usually refer to design documents, done by an architect or engineer specific to a particular project I'm working on, though the details can be applied to something like this and or many home projects, same is true if you can find some standard details.
6" x 6" woven wire mesh or W.W.M. as abbreviated and seen on most design documents is fine for a 4" slab which should suffice for the intended use you describe. If the edges are thicker like an Alaskan slab, probably ok to use #4 deformed bar (rebar), see if you can find a typical edge detail of this type of slab, 2 bars spaced at so many inches, stacked vertically or side by side. With that size slab, to create a reinforcing grid with #4 bar, will be expensive, makes the slab real strong, but probably overkill, use the 6"x6" W.W.M. on chairs, when placing concrete use a hook to tug up on the W.W.M., just make sure it's not bottomed out or too close to the top, 4" slab, you try to maintain 2" cover on each side, also if you are using anything corrosive, concrete has capillaries and it will find this W.W.M. react with it and cause it to expand 3x its original size and cause the concrete to fail, more prevalent on old bridge designs where salt is used for de-icing, good to match your design for it's use. Another thought is, what about future considerations, structural, say you want a vehicle lift, or heavy machine shop equipment, consider extra conduit, stub ups, and similar for future use or a spare, all these things are good to consider before, they cost a lot more later.
One comment about the brick used as a reinforcing chair from another poster, I'll agree, as my preference is to soak things anytime when placing concrete, you could soak the brick in a bucket, they hold quite a bit of water if you were to weigh them before or after, on that note, soak your sub-base, don't flood, but soak it, will help compaction and reduce the rapid absorption of water from the wet material, concrete will displace standing water anyway, just a preference whenever possible.
You also have many admixtures for concrete, one is fibrous reinforcing, someone else will have to comment on the use of that type of mix, have no experience with it.
Crack control is almost an art, but having those control joints strategically places may well do the trick, now if you use an expansion joint detail, make sure to use the appropriate expansion joint material, I'm seeing what appers to be recycled rubber, made into 1/2" by 4" wide strips, 10'-0" or 12'-0" long, you can recess or leave flush, recessed usually gets a sealant on top, there is a specific caulk for this.
Few cents worth,
PS, get your research done, prepare the job well in advance and plan for a comfortable pour, make sure you have able bodies to screed, place and finish, oh and I'd use a vibrating device to settle the material, just one quick pass and move with those, settles it in nicely, don't over do it. There is also another device with 2 handles like a walker on it and is perforated with holes, I think that would work just the same when packing and settling the fresh material, best of luck on it !
